Most of us can understand how we feel if someone else gets a better reward for doing the exact same work we did. Researchers are studying how these feelings of inequity evolved and if primates have the same sense of inequity.

After successful gene therapy, this male squirrel monkey that was once colorblind can now pick out red dots from the grey background. When the monkey correctly noses the red patch, a positive tone sounds and the monkey gets a drop of juice.